This invention relates to a flexible corrugated hose according to the preamble of the first claim.
Such flexible corrugated hoses find numerous applications, for instance in vacuum cleaners and other industrial applications. This and other types of hoses are often produced by blow-moulding a thermoplastic material, as this provides lightweight, flexible and inexpensive hoses. The wall of these hoses is provided with convolutions or corrugations, which results in added flexibility and crush and kink resistance. The corrugated shape makes the hose return to its original shape after being bent, compressed or stretched. Moreover, the corrugated shape prevents the hose from collapsing when the pressure inside the hose is reduced.
The corrugated shape of such a hose results in an internal surface with projections and recesses. It was found that the hose produces an annoying whistle when an air stream passes through its interior, especially at high rates of air flow. The whistle occurs when the hose is in stretched condition, but particularly when the hose is bent. The whistle is caused by resonance of audible harmonics which are created when the air, which is blown or sucked through the hose, strikes the corrugations of its internal surface.
From U.S. Pat. No. 4,756,045 a flexible hose of a plastic material is known which is produced by an extrusion blow-moulding process. The flexible hose disclosed in U.S. Pat. No. 4,756,045 has an internal surface comprising alternating projections and recesses, recesses of a greater depth and/or width alternating with recesses of a smaller depth and/or width. These alternating corrugations provide the hose with a roughened internal surface which reduces whistle by breaking up the flow of the passing air: the resonance caused by the harmonics created at the larger recesses is counteracted by the resonance caused by the harmonics created at the smaller recesses, which have a different frequency. This solution to the problem of unwanted whistle however has the disadvantage that the rougher internal surface amounts to a higher internal air resistance, giving rise to a loss of air pressure along the corrugations of the internal surface and thus to a loss of capacity of the hose.
There is thus a need for a flexible corrugated hose with which the problem of whistle can be solved without however increasing the internal air resistance of the hose. It is the object of this invention to provide a flexible corrugated hose which shows a reduced internal air resistance.
This is achieved according to the invention with the technical features of the characterising part of the first claim.
According to the invention each internal projection of the hose has a substantially flat and smooth top face. These flat top faces can prevent pressure loss on the internal protrusions and give the whole of the internal surface of the hose a smoother appearance, so that the air resistance of the interior of the hose can be reduced. By limiting pressure loss to the recesses and preventing pressure loss on the protrusions, the overall pressure loss over the entire length of the hose can be reduced.
As an air stream passes through the interior of the hose and the air strikes the side walls of the recesses, harmonics are created. Because of their substantially flat shape, the top faces form dampers for the harmonics which are created when air strikes the side walls of the internal recesses of the hose. The result is that a vibration created at one recess can be damped before the air reaches the next recess. This damping effect is supplied by the top face of the projection in between the two successive recesses. This damping effect has the advantage that less air resistance and less loss of air pressure occurs in the hose of the invention compared to the hose described in U.S. Pat. No. 4,756,045 in which the harmonics are counteracted by other harmonics of a different frequency. The damping effect in the hose of the invention reduces the amount of air vibrations created in the hose, whereas the counteracting effect in the hose described in U.S. Pat. No. 4,756,045 increases the amount of air vibrations created in the hose and causes comparatively more loss of air pressure.
An advantage of the smoother appearance of the internal surface of the hose is that less dust ends up in the internal recesses of the hose when it is used as a vacuum cleaner hose.
The ratio between the width of the top faces W1 and the width of the recesses W2 is preferably larger than or equal to 2.2, preferably W1/W2xe2x89xa73.5, in order to obtain that the damping effect exerted by the top faces can last long enough to sufficiently damp a vibration created at one recess before the air reaches the next recess. In this way interference of harmonics and a resulting production of whistle can be prevented. The first width W1 is the distance between a first intersection A of a first tangent line T1 with a second tangent line T2 and a second intersection B of a third tangent line T3 with the second tangent line T2. The second width W2 is the distance between the second intersection B and a third intersection C of a fourth tangent line T4 with a fifth tangent line T5. The first tangent line T1 touches a side wall of a first recess. The second tangent line T2 touches the top face of a first projection which is adjacent to the side wall of the first recess. The third tangent line T3 touches a first side wall of a second recess, this first side wall being adjacent to the first projection. The fourth tangent line T4 touches the second side wall of the second recess, this second side wall opposing the first side wall of this second recess. The fifth tangent line T5 touches the top face of a second projection which is adjacent to the second side wall of the second recess.
The inventor has also noted that the hose described in U.S. Pat. No. 4,756,045 and other existing hoses of a similar type are only substantially whistle-free at rates of air flow which do not exceed 115 m3/h. By providing a hose with a ratio W1/W2xe2x89xa72.2 according to the invention, preferably W1/W2xe2x89xa73.5, a hose is obtained which is still substantially whistle-free at higher rates of air flow, for example at 130 m3/h or higher. This is important because the rate of air flow of vacuum cleaners has risen from 90 m3/h in the past to 130 m3/h nowadays, so that the existing hoses are no longer applicable as they produce whistle at a rate of air flow of 130 m3/h. This feature of the hose of the invention can also be advantageous in other industrial appliances which involve a flexible hose with an air stream passing through its interior.
In order to obtain a hose with recesses which are as narrow as possible to prevent whistle especially at high flow rates, it is preferred that these recesses have substantially straight side walls which extend almost perpendicular to the longitudinal direction of the hose. In particular side walls are preferred of which the tangent line forms an angle of approximately 90 to 100xc2x0 with the plane formed by top faces of the internal projections.
According to another preferred embodiment of the invention the external surface of the hose comprises projections and recesses, higher projections alternating with lower projections and these lower external projections corresponding to the substantially flat top faces of the internal projections. These lower external projections are provided to collect an excess of plastic material and to prevent that plastic material piles up onto the projections in between two internal recesses of the internal surface of the hose during the moulding process. In other words, these lower external projections permit the plastic material to take such a shape that the internal projections have a substantially flat top face.
It is also preferred that along the internal recesses of the hose the wall has a substantially uniform material thickness T. Furthermore it is preferred that the transition between the top face of each internal projection and the side wall of each internal recess is formed by a substantially rounded edge. This rounded edge preferably has a curvature with a radius which is smaller than or equal to 1.5 times the material thickness T. This is preferred because the radius of the curvature of the rounded edges has to be small enough in order to prevent that the substantially flat area of the top faces of the internal projections becomes too small, which would result in a reduction of the damping effect of the top faces and interference of harmonics which are created when a passing air stream strikes the side walls of the internal recesses, thus producing of whistle.
The hose of the present invention is further preferably provided with a hollow longitudinal protuberance or score, in which for example one or more electrical conductors, a smaller tube containing conductors and possibly also a pull relief may be embedded, or which can be used for transporting a second gas or a fluid.
The present invention also relates to a mould for moulding a flexible hose as described above. Such a mould has a wall with a corrugated surface comprising in longitudinal direction alternating projections and recesses which extend in radial direction in the mould. The projections and recesses correspond to the projections and recesses of the external surface of the hose.
The mould is preferably provided with projections and deeper recesses alternating with less deep recesses, the deeper recesses extending further into the wall of the mould. At a given supply rate of the molten plastic material in the moulding process, the less deep recesses collect an excess of the plastic material urged against the projections of the mould, thus permitting the plastic material to form a hose with internal projections which have a substantially flat top face according to the invention. The less deep recesses are capable of preventing the plastic material from piling up and forming a hose with internal projections which have a convex shape, which is unwanted as this could result in hose with a higher air resistance along its internal surface, more loss of air pressure over its entire length and a production of whistle during operation.
The less deep recesses of the mould can have any shape, but in a preferred embodiment of the invention the less deep recesses have a substantially semicircular shape. The radius of this semicircular shape is preferably about equal to the material thickness T of the wall of the hose in order to obtain that the internal projections of the hose have a substantially flat top face, and not a top face which has a convex or concave shape.
The deeper recesses of the mould have a shape which permits the plastic material to flow easily into them. With this shape a hose of the desired strength and flexibility is formed and the removal of the hose after the moulding process is facilitated. To this purpose each of the deeper recesses of the mould preferably has two opposing side walls which form an angle of approximately 90 to 100xc2x0 with the longitudinal direction of the mould. Furthermore, each of these recesses preferably has a bottom width in the wall of the mould of about 2 to 3 and preferably about 2.5 times the desired material thickness T of the wall of the hose, in order to obtain a hose with recesses which are narrow enough to be able to prevent whistle and wide enough to ensure the flexibility of the hose. Also, the bottom corners of these deeper recesses preferably are slightly rounded, which can facilitate the removal of the mould from the hose. The depth of these deeper recesses is preferably about 1.5 times their bottom width in order to obtain a hose with sufficient strength.
It is preferred that the mould further has such a shape that the hose produced in the moulding process has internal projections with edges which are as little rounded as possible. To this purpose the transitions between the projections protruding from the wall of the mould and the side walls of each of the deeper recesses are preferably formed by sharp, not-rounded edges. The inventor has noted that providing the mould with such sharp, not-rounded edges does not adversely affect the strength of the hose at these edges, in other words does not lower the tear strength of the hose.
In one embodiment the mould has a wall provided with a longitudinal protuberance or score. In that way a hose can be produced which shows a corresponding hollow longitudinal score or protuberance, in which for example one or more electrical conductors, a smaller tube containing conductors and possibly also a pull relief may be embedded, or which can be used for transporting a second gas or a fluid.